1. Field of the Invention
This invention relates to a stabilized mask assembly for use in generation of thin film patterns and more particularly to a stabilized mask assembly having a dimensionally stabilized mask supporting frame having a relatively thin, substantially planar mask, such as for example a metal foil, rigidly affixed thereto under substantially uniform tension for direct deposition of thin film patterns on a substrate in a vacuum deposition process.
2. Description of the Prior Art
Utilization of patterns of thin films in microelectronics is well known in the art. Many methods for generating thin film patterns are known in the art including the use of film deposition masks which are commonly used in an evaporation vacuum deposition process.
Typically, a vacuum deposition mask is fabricated using known etching or electroforming techniques for forming a plurality of apertures in a well-defined pattern. In use, the deposition mask having the well-defined pattern is positioned in close proximity to a substrate upon which a thin film of material formed into the pattern of the well-defined pattern is to be deposited. An evaporant vapor from a vapor deposition source passes through the deposition mask and condenses on the substrate in the form of the well-defined pattern of the deposition mask to form the thin film layer of material.
In certain of the known prior vacuum deposition processes used for the fabrication of certain electrical components, such as for example, fabrication of thin film transducers or discrete electrical components such as helical coils, it is necessary to deposit a plurality of thin film layers of materials in various patterns onto a substrate wherein each deposited layer must be accurately located in an exact position on the substrate and in a predetermined relationship to the prior deposited layers on the substrate.
In connection therewith, it is also known in the art that placing a deposition mask relative to a vapor deposition source in a vapor deposition process results in the mask being subjected to heating due to the type of source being used in the vapor deposition process. Typically, the heat radiated to a mask from a resistively heated boat or crucible type of evaporation source is high. In comparison, the use of an electron beam type of evaporation source results in a lower amount of heat being radiated to a deposition mask.
In the known vapor deposition processes, a deposition mask is subject to heating from at least four (4) possible sources; namely, (a) radiation from the vapor source; (b) heat of vaporization released by the evaporant vapor condensing on the mask itself; (c) the heated substrate and substrate holder located proximate to the deposition mask, and (d) the heated mask support.
The known deposition masks have been formed from metal foil and the so-formed deposition masks are susceptible to changes in operating temperatures during the deposition process. Typically, the metal foil deposition masks have a thin cross-section and as a result thereof, exhibit low conductivity of heat and low total heat capacity. As such, the metal deposition mask is highly susceptible to temperature changes resulting from variations in (a) thermal emissivity and thermal conductivity due to the presence of evaporation material collected on the mask; (b) heat input to the deposition mask due to variations in the rate of condensation of evaporate material onto the deposition mask; and (c) thermal radiation from the source.
All of the above has the net effect of causing variations in the dimensions of the known deposition mask during the deposition process which results in an inability to precisely control registration and accurate dimensions of the resulting thin film deposited layer or layers.
Further as a result of the deposition mask being exposed to heating, it is known in the art that typically the deposition mask itself expands, the exact amount thereof being determined by the thermal expansion characteristics and the temperature change of the deposition mask itself.
The expansion of the deposition mask is known to have a thermal expansion effect which causes the well-defined pattern to be enlarged due to the expansion of the deposition mask at the operating temperatures of the deposition process.
Descriptions of the prior film deposition through masks and the methods and techniques for fabricating the same are disclosed in greater detail in a book entitled HANDBOOK OF THIN FILM TECHNOLOGY, Edited by Leon I. Maissel and Reinhard Glang, McGraw Hill Book Company, New York, New York, 1970, at Chapter 7 thereof entitled "Generation of Patterns in Thin Films" by Reinhard Glang and Lawrence V. Gregor at pages 7-1 to 7-10.
One technique for producing a thin film magnetic transducer using a vapor deposition process including deposition through a deposition mask is disclosed in U.S. Pat. No. 3,867,868 to Lazzari. In fabrication of a thin film transducer having pole piece layers and one or more winding layers, any one of several techniques can be used in an attempt to precisely control the width, length, depth and registration of the various layers relative to the prior deposited and post deposited layers. The apparatus utilized in fabricating such thin film transducers relied solely on the use of mask-carriage assembly which functionally attempted to index and accurately position a specific deposition mask pattern relative to a substrate.